Saturday, August 30, 2014

When and why does biology go quantum?

Here is my latest Crucible column for Chemistry World. Do look out for Jim and Johnjoe’s book Life of the Edge, which very nicely rounds up where quantum biology stands right now – and Jim has just started filming a two-parter on this (for BBC4, I believe).


“Quantum biology” was always going to be a winning formula. What could be more irresistible than the idea that two of the most mysterious subjects in science – quantum physics and the existence of life – are connected? Indeed, you get the third big mystery – consciousness – thrown in for good measure, if you accept the highly controversial suggestion by Roger Penrose and Stuart Hameroff that quantum behaviour of protein filaments called microtubules are responsible for the computational capability of the human mind [1].

Chemists might sigh that once again those two attention-grabbers, physics and biology, are appropriating what essentially belongs to chemistry. For the fact is that all of the facets of quantum biology that are so far reasonably established or at least well grounded in experiment and theory are chemical ones. The most arguably mundane, but at the same time the least disputable, area in which quantum effects make their presence felt in a biological context is enzyme catalysis, where quantum tunneling processes operate during reactions involving proton and electron transfer [2]. It also appears beyond dispute that photosynthesis involves transfer of energy from the excited chromophore to the reaction centre in an excitonic wavefunction that maintains a state of quantum coherence [3,4]. It still seems rather staggering to find in the warm, messy environment of the cell a quantum phenomenon that physicists and engineers are still struggling to harness at cryogenic conditions for quantum computing. The riskier reaches of quantum biology also address chemical problems: the mechanism of olfaction (proposed to happen by sensing of odorant vibrational spectra using electron tunneling [5]) and of magnetic direction-sensing in birds (which might involve quantum entanglement of electron spins on free radicals [6]).

Yet it is no quirk of fate that these phenomena are sold as a union of physics and biology, bypassing chemistry. For as Jim Al-Khalili and Johnjoe McFadden explain in a forthcoming comprehensive overview of the field, Life On the Edge (Doubleday), the first quantum biologists were pioneers of quantum theory: Pascual Jordan, Niels Bohr and Erwin Schrödinger. Bohr was never shy of pushing his view of quantum theory – the Copenhagen interpretation – into fields beyond physics, and his 1932 lecture “Light and Life” seems to have been influential in persuading Max Delbrück to turn from physics to genetics, on which his work later won him a Nobel Prize.

But it is Schrödinger’s contribution that is probably best known, for the notes from his lectures at Trinity College Dublin that he collected into his little 1944 book What Is Life? remain remarkable for their prescience and influence. Most famously, Schrödinger here formulated the idea that life somehow opposes the entropic tendency towards dissolution – it feeds on negative entropy, as he put it – and he also argued that genetic information might be transmitted by an arrangement of atoms that he called an “aperiodic crystal” – a description of DNA, whose structure was decoded nine years later (partly by another former physicist, Francis Crick), that still looks entirely apt.

One of the most puzzling of biological facts for Schrödinger was that genetic mutations, which were fundamentally probabilistic quantum events on a single-atom scale, could become fixed into the genome and effect macroscopic changes of phenotype. By the same token, replication of genes (which was understood before Crick and Watson revealed the mechanism) happened with far greater fidelity than one should expect from the statistical nature of molecular interactions. Schrödinger reconciled these facts by arguing that it was the very discreteness of quantum events that gave them an accuracy and stability not amenable to classical continuum states.

But this doesn’t sound right today. For the fact is that Schrödinger was underestimating biology. Far from being at the mercy of replication errors incurred by thermal fluctuations, cells have proof-reading mechanisms to check for and correct these mistakes.

There is an equal danger that quantum biologists may overestimate biology. For it’s all too tempting, when a quantum effect such as tunneling is discovered in a biological process, to assume that evolution has put it there, or at least found a way to capitalize on it. Tunnelling is nigh inevitable in proton transfer; but if we want to argue that biology exploits quantum physics here, we need to ask if its occurrence is enhanced by adaptation. Nobel laureate biochemist Arieh Warshel has rejected that idea, calling it a “red herring” [7].

Similarly in photosynthesis, it’s not yet clear if quantum coherence is adaptive. It does seem to help the efficiency of energy transfer, but that might be a happy accident – Graham Fleming, one of the pioneers in this area, says that it may be simply “a byproduct of the dense packing of chromophores required to optimize solar absorption” [8].

These are the kind of questions that may determine what becomes of quantum biology. For its appeal lies largely with the implication that biology and quantum physics collaborate, rather than being mere fellow travellers. We have yet to see how far that is true.

1. R. Penrose, Shadows of the Mind (Oxford University Press, 1994).
2. A. Kohen & J. P. Klinman, Acc. Chem. Res. 31, 397 (1998).
3. G. S. Engel et al., Nature 446, 782 (2007).
4. H. Lee, Y.-C. Cheng & G. R. Fleming, Science 316, 1462 (2007).
5. L. Turin, Chem. Senses 21, 773 (1996).
6. E. M. Gauger, E. Rieper, J. J. L. Morton, S. C. Benjamin & V. Vedral, Phys. Rev. Lett. 106, 040503 (2011).
7. P. Ball, Nature 431, 396 (2004).
8. P. Ball, Nature 474, 272 (2011).

1 comment:

John mery said...

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